In his Knowledge and its Limits (2000) Timothy Williamson argues that knowledge can be causally efficacious and as such figure in psychological explanation. His argument for this claim figures as a response to a key objection to his overall thesis that knowing is a mental state. In this paper I argue that although Williamson succeeds in establishing that knowledge in some cases is essential to the power of certain causal explanations of actions, he fails to do this in a way that establishes knowledge itself as a causal factor. The argument thus fails to support his overall claim that knowledge should be conceived as a state of mind